An air brake system for a vehicle such as a bus, truck or the like typically includes a brake actuator assembly which is actuated by means of an actuator assembly operated by the selective application of a fluid such as compressed air. Conventional air brake actuators typically have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency or spring brake actuator which causes actuation of the brakes when air pressure has been released. The spring brake actuator can be used as a parking brake or emergency brake in the event that the air pressure system fails. The spring brake actuator includes a compression spring which forces application of the brake when fluid or air pressure is either released or lost. Typically, the spring brake actuator is disposed in tandem with the service brake actuator.
The service chamber which houses the service break actuator is typically divided into two chambers by a diaphragm. Depressing the brake pedal during normal driving operation introduces compressed air or fluid into one of the chambers of the service brake actuator which, acting against the diaphragm, causes a service brake push rod in the opposite chamber to be extended and the brakes to be applied with an application force proportional to the air pressure in the service brake actuator.
Like the service chamber, the spring brake actuator is typically divided into two chambers, a pressurized chamber and a non-pressurized chamber, separated by a rubber diaphragm and pressure plate, with the spring in the non-pressurized chamber acting between an end wall of the spring brake housing and the pressure plate. When compressed air or fluid is introduced to the pressurized chamber, air or fluid pressure acting against the diaphragm and pressure plate compresses the spring in the non-pressurized chamber.
In the event of a loss of air or fluid pressure or an intentional exhaustion of air or fluid from the spring brake actuator, the spring brake will be mechanically activated by the force of the compression spring acting on the spring brake actuator rod which, in turn, acts upon the brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake. Because the force generated by the compression spring is quite large, it can be dangerous to service the spring brake if the spring has not been properly restrained. For safety purposes during servicing, some spring brake actuators include a release bolt and nut assembly that can be adjusted to retain the spring in a fixed and safe position during surfacing of the brake.
When air or fluid pressure is released from the spring brake actuator, the spring and diaphragm extend significantly, expanding the volume of the spring brake actuator non-pressurized chamber containing the spring. A pressure vacuum is then created in the chamber by the expanding volume of the non-pressurized chamber, so means must be provided to allow air to enter into the expanded volume of the non-pressurized chamber from the outside environment. Conversely, when the spring is retracted, and the volume of the non-pressurized chamber contracts, means must be provided for evacuating air from the chamber.
In many prior brake actuators, the chamber containing the spring is simply open to the outside environment and atmosphere through ports or vents in the chamber housing. However, this allows dirt, salt, moisture and other unwanted material and contaminants from the environment outside the brake chamber to enter that chamber through the ports. Some prior art brake designs have used complex arrangements of valves and fluid conduits to permit fluid flow to the spring brake chamber from other chambers in the brake actuator assembly. These arrangements require extra components and complex machining of the components. Other prior art brake actuators have used external filters placed over the vent holes and attached to the outside of the brake actuator housing to prevent contaminants from entering the brake chambers. However, external filters require additional space, components, machining and assembly effort.
Prior art brake actuators include U.S. Pat. No. 5,937,773 which discloses a spring brake actuator with an internal breathing conduit made up of a series of complicated weep holes in the spring brake housing, the spring brake actuator rod and the sleeve of the push rod. This series of weep or vent holes fluidly connects the unpressurized chambers of the brake with the outside environment. The fluid which enters and exits the brake actuator through the external vent hole passes through a filter assembly that attaches to the exterior of the spring brake housing and covers the external vent hole filtering fluid passing in and out of the spring brake.
U.S. Pat. No. 5,632,192 issued to Plantan et al. discloses a spring brake actuator which includes an indicator system incorporated into a release tool therefor. The indicator provides an indication of when the release tool has is fully released. The release tool is of the type wherein a threaded member does not extend outwardly of the actuator during caging of the power spring. The indicator is biased outwardly of the actuator when the release bolt has even partially caged the power spring. It is only when release bolt is fully released that the indicator is pulled inwardly. Thus, an observer has a visual indication of when the power spring caging mechanism has been fully released. However, this reference does not disclose any type of filter or vent system at all.
U.S. Pat. Nos. 5,263,403 and 5,311,809 issued to Choinski et al. disclose a brake actuator having a non-pressurized spring brake chamber and a non-pressurizing service brake chamber fluidly interconnected by a breather tube mounted on the outside of the brake actuator housing.
U.S. Pat. No. 4,890,540 issued to Mullins discloses a brake actuating unit having a housing with a pair of vent holes wherein the lower vent hole is left completely open to the outside environment without a cap or filter while only the upper vent hole is sealed to prevent contaminants from entering the housing.
U.S. Pat. No. 3,896,706 issued to Newstead discloses a brake unit having vent holes in the brake housing of the non-pressurized chamber that are covered by external filters that are attached to the outside of the brake housing.
U.S. Pat. No. 5,123,330 issued to Roether et al, U.S. Pat. No. 4,283,992 issued to Wilson and U.S. Pat. No. 4,259,895 issued to Owens all disclose assemblies having filters for outside air located within or adjacent to actuating cylinders.
All of the prior art references listed above suffer from a number of disadvantages, including the fact that complex filter designs are employed, that additional holes are required to be created in the housing, and/or that unsatisfactory filtering of contaminants is achieved.